My problem with that is this: if there are indeed only 9 wizard info schools in the world, and by Leonardo's estimate, they sustain a population of 2500-3000 each, there is no way that stadium could be filled with 100,000 wizards. Even if we were to assume that the average school wasfour times as large as Hogwarts, to fill the stadium, nearly the entire wizarding population would need to be there. And, as has been said before, tickets were not exactly cheap.
The only feasible way I could see would be that Britain has a specific reason its wizard density is lower than elsewhere (if it wasn't, then there would only be like 250K wizards worldwide, and I don't exactly think 40% of world wizards would have been there), causing them to only need the one major school while other countries do have minor schools. Perhaps due to some sort of persecution in the past that occurred in Britian and a few other countries only?)
Or JK Rowling just sucked at math.